§1.1 Field of the Invention
The present invention concerns data communications. In particular, the present invention concerns relaying information in a wireless local area network (LAN) cooperatively.
§1.2 Background Information
As an advanced broadband wireless access technology, Worldwide Interoperability for Microwave Access (“WiMAX”) has attracted a lot of research attention. While the current IEEE 802.16e standard (See, e.g., “IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems, Amendment 2 and Corrigendum 1,” IEEE 802.16e (February 2006) incorporated herein by reference) has been specified for a single-hop WiMAX network, relay-assisted WiMAX has been considered for the future evolution of WiMAX standards, and is being actively investigated. (See, e.g., “Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems: Multihop Relay Specification,” IEEE Baseline Document for Draft Standard for Local and Metropolitan Area Networks (April 2007) incorporated herein by reference.)
The IEEE 802.16j baseline draft proposed the concept of WiMAX mobile multi-hop relay (“MMR”) system. A standard MMR system enables a subscriber station (“SS”) to route its sent data through intermediate relay stations (“RSs”) in order to reach the base station (“BS”). In the MMR scenario, cooperative communication is considered as an efficient solution in multi-hop transmissions because it provides robust forwarding by recruiting multiple intermediate stations to collaboratively transmit the source signal to the destination. (See, e.g., the articles: A. Sendonaris, E. Erkip, and B. Aazhang, “User Cooperation Diversity—Part I: System Description,” IEEE Transactions on Communications, Vol. 51, No. 11, pp. 1927-1938 (November 2003); A. Sendonaris, E. Erkip, and B. Aazhang, “User Cooperation Diversity—Part II: Implementation Aspects and Performance Analysis,” IEEE Transactions on Communications, Vol. 51, No. 11, pp. 1939-1948 (November 2003); and J. N. Laneman, D. Tse, and G. Wornell, “Cooperative Diversity in Wireless Networks: Efficient Protocols and Outage Behavior,” IEEE Trans. on Info. Theory, Vol. 50, No. 12 (December 2004), each of which is incorporated herein by reference).
To increase the spatial diversity gain, SSs and RSs can be used as the intermediate stations to form a virtual multi-input multi-output (“MIMO”) system. These RSs and SSs are called “helpers” and act as distributed antenna array elements. For a virtual MIMO system, space-time coding (“STC”) is used in a distributed fashion, commonly known as distributed spacetime coding (“DSTC”), so as to achieve high diversity gains in a cooperative environment. The basic idea of DSTC is to coordinate and synchronize helpers so that each of them acts as one antenna of a conventional STC. (See, e.g., J. N. Laneman et al., IEEE Trans. on Info. Theory, Vol. 50, No. 12 (December 2004.)
Although DSTC is being investigated by the IEEE 802.16j/16m standard task group as a possible virtual MIMO technique (See, e.g., W. Ni, G. Shen, and S. Jin, “Cooperative Relay Approaches in IEEE 802.16j,” IEEE C802.16j-07/258r 1, IEEE 802.16 Broadband Wireless Access Working Group (April 2007), incorporated herein by reference), it still has the following inherent drawbacks:                1) Each helper participating in a DSTC needs to be numbered so that it knows exactly which antenna it will mimic in the underlying STC. Hence the exact set of participating helpers needs to be known and distributed to the network using signaling messages;        2) Whenever rate adaptation is used, DSTC relies on detailed global channel state information between each potential helper and the source as well as the destination. The dissemination of such information leads to MAC layer signaling, which could be very costly in a mobile environment. Outdated channel state information often results in severe loss in performance;        3) Even though nodes other than the chosen helpers may decode the source information correctly, they are not allowed to transmit. This sacrifices potential diversity and coding gain; and        4) A DSTC relies on tight constraints on the time synchronization of the nodes, putting a very heavy burden on the MAC and PHY layers.        
These issues can be addressed by employing Randomized Distributed Space-time Coding (“R-DSTC”). (See, e.g., the articles: B. S. Mergen and A. Scaglione, “Randomized Space-Time Coding for Distributed Cooperative Communication,” IEEE Transactions on Signal Processing, pp. 5003-5017 (October 2007); F. Verde, T. Korakis, E. Erkip, and A. Scaglione, “On Avoiding Collisions and Promoting Cooperation: Catching Two Birds with One Stone,” IEEE SPAWC, (July 2008) and U.S. patent application Ser. No. 12/938,101, titled “Robust Cooperative Relaying in a Wireless LAN: Cross-layer Design and Performance Analysis,” filed on Nov. 2, 2010 and listing Elza ERKIP, Pei LIU, Chun NIE and Shivendra S. PANWAR as inventors, each of which is incorporated herein by reference.) R-DSTC is a novel DSTC technique with the advantages of robustness and simplicity. While the PHY layer principles of the R-DSTC technique have previously been studied, there is a need for improvement in the application of the R-DSTC technique in practical wireless networks. A preliminary MAC layer effort is presented in the article P. Liu, Y. Liu, T. Korakis, A. Scaglione, E. Erkip, and S. Panwar, “Cooperative MAC for Rate Adaptive Randomized Distributed Space-Time Coding,” Proc., IEEE Globecom (November 2008) (incorporated herein by reference) and initial crosslayer design issues are explored in the article F. Verde, et al., IEEE SPAWC (July 2008). However, these papers do not adapt the R-DSTC protocol design to WiMAX networks. Crucially, the additional overhead required at the MAC layer needs to be quantified in order to understand the overall gains from this PHY layer technique.
Thus, it would be useful to provide an MMR system that is applicable to various practical wireless networks (such as an IEEE 802.16 (WiMAX) network for example) and that avoids the drawbacks of DSTC.